1. Field of the Invention
The present invention relates to an image forming apparatus such as an electrophotographic copying machine and a printer in which the surface of a photoreceptor drum is charged by a main charger unit, and more particularly, to an image forming apparatus having a function to maintain a main charging voltage of the main charger unit to be low.
2. Description of the Prior Art
In an image forming apparatus such as an electrophotographic copying machine and a laser beam printer, to form an electrostatic latent image on the surface of a photoreceptor drum by means of an exposing portion, the drum surface is charged in advance by a main charger unit of Scorotron type including a grid electrode and a high-voltage electrode for applying a main charging voltage. At the time of the charging, the grid voltage is fixed to a predetermined value, and the main charging voltage is varied so that a desired surface potential is obtained at a main charging voltage-to-photoreceptor drum surface potential characteristic at the grid voltage. However, in such a conventional surface potential controlling arrangement, since the main charging voltage-to-photoreceptor drum surface potential characteristics is fixed to one because of the fixed grid voltage, in order to obtain a high surface potential at the characteristic, it is necessary to apply a very high main charging voltage. However, it is difficult to supply such a high main charging voltage stably, and an environmental problem such as the generation of a large quantity of ozone is caused.